Jinpei, codenamed "the Swallow" and designated G-4, holds the distinction of being the Science Ninja Team's youngest member. He excels as a reconnaissance expert and infiltrator, leveraging his small stature and agility for intelligence gathering, yet readily engages in combat when mission demands dictate. Opponents frequently underestimate him due to his youth, a vulnerability he actively counters. Orphaned in infancy, Jinpei was discovered by Jun within the Jupiter Mountains. She brought him to the Shirayuki Orphanage, where they were raised together, forging a deep sibling bond. Dr. Nambu later recruited both for the Science Ninja Team, with Jun securing their joint participation as a condition. Jinpei asserts he is the final descendant of the Iga Ninja clan, and hints suggest he might be the sole surviving son of the Jupiter Ninja Clan's sensei, though this remains unverified. His personality blends cheerful mischievousness with a sense of responsibility. He assists Jun in running her snack bar, Snack J, displaying business savvy and culinary talent. Despite a dislike for academic subjects like math, he possesses technical aptitude, notably demonstrated in the OVA where internet-based education grants him gaming prowess and computer hacking expertise, facilitated by a specialized hacking device integrated into his helmet. He exhibits a habit of rescuing stray animals, sometimes inadvertently triggering team missions. Relationships form a core aspect of his character. Jun is his adoptive sister and primary caregiver, initially addressed as "Onēchan." Ken is viewed as an older brother figure ("Aniki"), while Joe also holds a fraternal role. Ryu serves as his comedic partner. Dr. Nambu acts as a father figure overseeing his education. Jinpei undergoes notable development across episodes, confronting feelings of being undervalued due to his age (episode 8), briefly experiencing maternal care from a village woman (episode 69), and exploring his potential ninja heritage on Mount Jupiter (episode 95). His appearance evolves across adaptations. The original series features civilian attire with an orange-and-yellow "4" T-shirt and blue-and-white striped bellbottoms, and a yellow Bird Style uniform with red gloves/boots and a blue cape. The OVA modernizes his civilian look with a beanie and sweatshirt, while his Bird Style shifts to primarily red with yellow accents and a purple cape. Concept art for the cancelled Imagi film depicts varied designs, including dark blond hair and different outfits. His weapons also change: American Clackers (bolas) in the original series, a Rebound Ball projectile in *Gatchaman II*, and a Swallow Top (yo-yo-like spinning top) in *Fighter*. His mecha include the reconnaissance-focused Swallow Helico and the tank-like Gatcha Four, contributing to the combined Gatcha Spartan in *Fighter*. International adaptations significantly alter his character. *Battle of the Planets* renames him Keyop, portraying him as a genetically engineered child with a speech impediment, a concept absent from the original material.